The project examining the effect of serum lipoproteins, and other proteins transporting thyroid hormones in serum (TBG, TTR and HSA), on the efflux of the hormones from cultured human fibroblasts and hepatocytes has been completed and published. It was concluded that none of these proteins specifically facilitated thyroid hormone efflux but acted instead as sinks for the hormones outside the cells. It was of interest, however, that the lipoproteins were more active in this process than serum albumin although the affinity constants were similar. This suggested that, owing to their lipid content, the lipoproteins were able to associate more intimately with the cell membrane.